Peter Madsen said in court: ‘There is nothing worse that can happen to a person like me than to sail out with another person and return alone.’
Photograph: Mikko Suominen/Getty Images

A Danish inventor has denied murdering the Swedish journalist Kim Wall on his homemade submarine, claiming that he had lied previously about how she died only to spare her relatives.

“I deny that I am guilty of the murder of Kim Wall,” Peter Madsen told the court in Copenhagen on the first day of his trial, after prosecutor Jakob Buch-Jepsen had presented a barrage of new detail.

Madsen said he had claimed up until October that Wall had been killed by a heavy latch falling on her head to avoid making public the “grim” reality of her death.

“I was only thinking about Kim Wall’s relatives,” he said. “It’s a very, very horrible story. I knew she was dead and she would not come back. I had only one purpose and it was to spare the relatives as much as possible.”

About 120 journalists from 15 different countries crowded into the court and a media room at Copenhagen district court, reflecting the huge international interest in the case.

Wall, an award-winning journalist, joined Madsen on the afternoon of 10 August, hoping to interview him about his nine-year push to send himself into suborbital space. Her torso was spotted 11 days later off the coast of Copenhagen, and her head, arms and legs were found by police divers over the following months.

During his questioning in court, Madsen said a technical fault had led to exhaust gases being pumped into the submarine’s “mess”, poisoning Wall while he was trapped outside, the negative pressure preventing him from opening the hatch.

“I tried to shout to Kim that she should try to turn off the engines,” he said. “I do not know if Kim succeeded, but the engines stopped.”

That released the pressure, allowing him to open the hatch, he said, whereupon he found her lifeless body.

Buch-Jepsen thoroughly discounted Madsen’s story during his morning overview, when he said such an accident would have led to rapid heating, for which there was no evidence. A technical report will be presented later in the trial.

The trial opened with new forensic evidence, as the prosecution began presenting its case that Madsen killed Wall, 30, to fulfil his violent sexual fantasies, in an act planned days or even months beforehand.

Buch-Jepsen told the court that no traces of Madsen’s DNA had been found on the body of the Swedish journalist. “But traces of semen have been found in the underpants which were secured from Peter Madsen after the arrest,” he said. Under questioning, Madsen denied ejaculating while he was with Wall.

Madsen, 47, dressed in a black T-shirt, spent the morning calmly leafing through the prosecution’s papers, as Buch-Jepsen ran through evidence including photographs and diagrams of the stab wounds on Wall’s body.

Madsen’s denial of guilt on Thursday afternoon were his first words in court since police divers found Wall’s head in October, disproving his earlier claim that a heavy latch had smashed her skull.

In a detailed presentation that lasted about two and a half hours, Buch-Jepsen said flecks of Kim Wall’s blood had been found on Madsen’s nostrils, and fresh scratches on his underarms.

He said Madsen had, in the months leading up to Wall’s death, searched the internet for videos of women being beheaded. Police, he said, had managed to recreate Madsen’s iPhone from backups right up until the day of the submarine trip on 10 August.

Madsen had used it to carry out mobile searches for “throats”, “haircut”, “girl” and “pain”, with the last search made in the early morning before Wall joined him on his submarine.

Buch-Jepsen also showed the court pictures of Wall’s arms, saying a strap with a ratchet buckle found on the submarine matched marks on her wrists.

He showed pictures of Madsen’s workshop months before the event, pointing out a saw that he said police divers later found in Køge Bay, south of Copenhagen, close to the plastic bag holding Wall’s head.

An investigation by forensic psychiatrists had determined, he continued, that Madsen had “narcissistic and psychopathic traits” showing a “severe lack of empathy, remorse and guilt”, despite a “superficial charm”.

The trial will be held over 12 days in March and April, with the verdict expected on 25 April, but may be delayed if days are postponed due to a planned strike by public sector workers.